The present invention relates to a cold trap for retaining impurities which can be encountered in the liquid sodium of a supply circuit, e.g. of the type found in the nuclear industry.
The impurities found in the liquid sodium of such a circuit can have various origins and can in particular result from the initial sodium charge, contamination by the confinement materials of the vessels containing the same, gases covering said vessels, pollution resulting from the accidental entry of air particularly during maintenance or assembly work on the installation in which the sodium circulates, the products of a possible reaction between the sodium and the water and finally the diffusion of hydrogen or hydrogenated products through the walls of the heat exchangers or steam generators of a heat transfer installation.
All these impurities must be permanently removed as soon as they appear by precipitation - filtration -purification because they have a prejudicial influence on:
the precipitation of the compounds formed with the sodium in the cold branches of the circuits,
the embrittlement of steels (C, N.sub.2, H.sub.2),
the acceleration of corrosion (O.sub.2, H.sub.2, NaOH),
the increase in the residual hydrogen concentration disturbing the satisfactory operation of the leak detection of the circuit of the installation by measurement of the hydrogen concentration.
In an exemplified manner, FIG. 1 shows a known cylindrical cold trap for illustrating the construction and operation of such an apparatus.
This known control trap essentially comprises a vertically axed, cylindrical reservoir 1, in which circulates the sodium to be purified and coming from the confinement enclosure (circuit or vessel). In its upper part 2, the cold trap has an economizer 3, constituted by usually helically wound tubes, into which passes the hot liquid sodium to be treated via pipe 4. In said upper part 2, the temperature of the hot sodium (approx. 400.degree. C.) is progressively lowered to approach the cold point temperature (110.degree. to 120.degree. C.) of the installation. The sodium flows from the economizer via a series of jets such as 5 directed towards the low part 6 of the installation. This low part essentially has an annular filter plug 7, generally formed by stainless steel wool, in which the sodium flows from top to bottom in accordance with the arrows F and then rises again, as shown by the arrows F', in the central inner shaft 8 and leaves via pipe 9, after reheating to approximately 360.degree. C.
The low part 6 is cooled and maintained at the cold point temperature with the aid of a cooling flow diagrammatically indicated by arrows F" in FIG. 1, which indicate that a cooling liquid or gas flows outside reservoir 1 and on low part 6 thereof in order to reduce its temperature. This fluid arrives at a temperature of 20.degree. C. and is then heated to e.g. approximately 80.degree. C. The sodium circulating between the filter plug 7 is also maintained at the cold point temperature of the trap and deposits its impurities, which are both condensed and filtered during passage through the filter plug 7.
The treated and purified sodium, which rises through the inner shaft 8 then flows towards the outside through pipe 9, after passing through the high part 2 of the trap in the opposite direction.
Thus, in the trap of FIG. 1, all the sodium flow from the cold trap passes through the cold point before rising through the inner shaft 8 up to the high part of the trap, where it is reheated in economizer 3 to a temperature close to its entry temperature. Thus, this economizer 3 functions as a heat exchanger between the hot sodium entering via pipe 4 and the cold sodium leaving via pipe 9.
In a cold trap of the type described relative to FIG. 1, the lower part is generally cooled by a liquid or gaseous fluid, e.g. an organic liquid or air circulated by a not shown pump and the heat flow to be removed then passes through the outer envelope or wall of reservoir 1.
Although this construction functions completely satisfactorily, it only permits a relatively limited cooling of the filter plug 7, because the heat to be removed must pass through the outer envelope, which has a relatively reduced surface area. When it is wished to obtain a high cooling flow, for a large sodium flow rate (large capacity trap), the cooling medium is generally constituted by an organic liquid, whose calorific capacity is higher than that of a gas. However, as stated hereinbefore, this means that it is necessary to have a circuit for the organic liquid and which comprises a heat exchanger and pump, which makes the operation of the trap much more difficult.
When using a small capacity trap, it is merely necessary to cool its lower part with air, but then the heat flow which can be removed is limited by the small exchange coefficient between the air and the trap wall. The heat exchange flow can be increased to a limited extent by increasing the exchange surface, which is then lined with transverse or longitudinal blades and/or barbs and wires welded to the outer ferrule.
However, cylindrical traps of the type shown in FIG. 1 have a treatment capacity which is relatively limited as a result of the small exchange surface.